Hitherto, as a method for producing fluoroalkanesulfonic anhydrides, when citing as an example a method for producing trifluoromethanesulfonic anhydride ((CF3SO2)2O), there has been known a method for producing the same by a dehydration condensation reaction by using trifluoromethanesulfonic acid (CF3SO3H) as a starting raw material and by adding diphosphorus pentoxide (P2O5) to trifluoromethanesulfonic acid as shown in the following reaction formula 1. In the case of using this method, the trifluoromethanesulfonic anhydride formed is recovered by evaporation by heating.2CF3SO3H+P2O5→(CF3SO2)2O+P2O5.H2O  Reaction Formula 1:
In the present method, however, metaphosphoric acid (P2O5.H2O) produced as a by-product by the dehydration condensation reaction of trifluoromethanesulfonic acid is glassy to have a very high viscosity. Thus, stirring becomes impossible in the middle of the reaction by using one-shaft blades, stirring-type reactor that is generally used for mixing and stirring high viscosity materials. Therefore, the reaction does not proceed, and it also becomes impossible to recover the trifluoromethanesulfonic anhydride formed. Thus, yield of trifluoromethanesulfonic anhydride becomes 60% or lower at the maximum, based on trifluoromethanesulfonic acid.
Thus, there is known a method (Patent Publication 1) for recovering the unreacted trifluoromethanesulfonic acid by conducting again a distillation under reduced pressure after having a condition that stirring of the reactor has been made possible by dissolving metaphosphoric acid by adding water or phosphoric acid to this glassy reaction residue.
In this method, however, it is necessary to stop the reaction once and gradually add water or phosphoric acid to avoid a rapid temperature increase by the reaction heat. Furthermore, it is not possible to conduct stirring either until it dissolves. Therefore, it is necessary to wait for long hours until metaphosphoric is completely dissolved in the added water or phosphoric acid. Viewing it from the industrial viewpoint, it is difficult to consider that the method is necessarily a process superior in productivity.
On the other hand, in order to correct the low yield by solidification of metaphosphoric acid, it is known that solidification by metaphosphoric acid during the reaction can be suppressed, by a method (Patent Publications 2 and 3) in which trifluoromethanesulfonic acid and diphosphorus pentoxide are reacted in a fluorine-series solvent, a method (Patent Publication 4) in which trifluoromethanesulfonic acid is used in an excessive amount relative to diphosphorus pentoxide, and the like. However, in order to obtain a viscosity at the level of being able to maintain at least stirring during the reaction, it is necessary to use a large amount of solvent or a large amount of the raw material that is not consumed by the reaction. Therefore, viewing from the industrial viewpoint, it is difficult to consider that the method is necessarily a process superior in productivity.
In contrast with the above-mentioned method, as a method for producing a fluoroalkylsulfonic anhydride with a high yield, there is known a method (Patent Publication 5) in which kneading is compulsorily conducted even under a condition of solidification by metaphosphoric acid to continue the reaction, by reacting the fluoroalkylsulfonic acid with diphosphorus pentoxide by using a kneader-type reactor equipped with two-shaft blades.
Furthermore, it is known that kneader-type reactors are used in the method for producing polysaccharides (Patent Publication 6) and the method for producing geleous polymerizates (Patent Publication 7).